Wouldn't do shit for balance because items are necessary for certain champions to function, for example GP is pretty useless without items (he'd be a squishy, mediocre DPS, decent teamfighting champion, due to ok DPS from constant Q's and barrels and ult AoE, however his barrels would not be his main source of damage like they are now.
Many mages also need mana items to do anything, so most of them would seem way too weak without items, but still might be OP with items.
Assassins would rise to the top, with their inbuilt juking and damage blocking they'd still be just as slippery as they are now, and they'd abuse their medium cooldown high scaling abilities because fights would last long enough for them to get off multiple rotations.
Riven would be ok, her high base damages making her mobile and still powerful, and she would not fall off lategame without strict combo'ing because her enemies can't delete her and she'll have time for a full fast combo. On the otherhand, oneshot combo's would not do enough damage to oneshot anyone.
That being said, lack of CDR means her DPS will be utter shit.
Darius would remain very powerful unless faced with multiple opponents or hard kited, his high base damages and his ability to utilize his passive in the extended fights (no melee would have the burst to get through his base stats) that would happen without items means his damage will stay very effective.
People wouldn't complain about Fiora.
Renekton would fall off much later, maybe never. His ult gives him a shit ton of tank stats not to mention his high base damages. Extended fights means he gets more fury empowered abilities.
Jax would scale well into mid and late game, and probably also have one of the strongest early games (after first back). Although he might go oom before killing you and would have a harder time sticking.
Quinn would fall off tremendously after first back, her base damages are terrible. W steroid means she could kite, but it would take her forever to kill anyone.
: Everyone is one-shot
: I agree
are pretty good without items.
just happens to have all 3